What is a grass seed calculator?
A grass seed calculator tells you how much seed to buy for a lawn. Seed bags are labelled with a seeding rate — a weight of seed per unit of ground — and that rate depends on two things: the grass species you are sowing and whether you are starting a lawn from bare soil or thickening one that already exists. The calculator takes the lawn’s length and width, multiplies them into an area, applies the rate for the grass type and purpose you selected, and returns the seed weight you need.
Sowing too little seed leaves thin turf that weeds colonise before the grass fills in. Sowing far too much is not a shortcut either: seedlings that germinate shoulder to shoulder compete with each other for water and light, and the stand ends up weak and disease-prone. Buying to the published rate is the reliable middle.
How does the calculator work?
Enter the lawn length and lawn width — each has its own unit selector, so you can measure in feet, yards, meters, centimetres or inches — then choose the grass type and the seeding purpose (new lawn or overseeding). The calculator computes the lawn area, looks up the seeding rate, and shows two results:
- Total lawn area, switchable between ft², yd², m², acres and hectares.
- Grass seed needed, switchable between lb, kg, oz and g.
For an L-shaped or irregular lawn, split it into rectangles, run each one through the calculator, and add the seed weights together.
Seeding rates
Seeding rates are published in pounds of seed per 1,000 square feet, the unit used on seed bags and in extension bulletins. The figures below are the consensus rates from US cooperative-extension turfgrass guidance (Penn State Extension’s lawn establishment guidance, Purdue Extension AY-3, and University of Maryland Extension lawn seeding tables all publish the same numbers). Overseeding uses about half the new-lawn rate, because the established turf already occupies most of the ground and the new seed only has to fill the gaps.
| Grass type | New lawn (lb/1,000 ft²) | Overseeding (lb/1,000 ft²) | New lawn (g/m²) |
|---|---|---|---|
| Kentucky bluegrass | 2 | 1 | 9.8 |
| Tall fescue | 8 | 4 | 39.1 |
| Perennial ryegrass | 8 | 4 | 39.1 |
| Bermuda grass | 2 | 1 | 9.8 |
| Fine fescue | 4 | 2 | 19.5 |
The spread between species is mostly a matter of seed size. Kentucky bluegrass and Bermuda have tiny seeds — well over a million of them per pound — so two pounds already puts plenty of seed on the ground. Tall fescue and perennial ryegrass have much heavier seeds, so the same seed count weighs four times as much.
These rates are estimates for typical conditions. They are the right starting point for a lawn, but the rate printed on the bag of the specific cultivar or blend you buy takes precedence, and a seed blend’s rate reflects the mix rather than any single species in this table.
Formulas
The lawn area is length multiplied by width. With length and width :
The seed weight is the area in thousands of square feet, multiplied by the seeding rate (in pounds per 1,000 ft²) for the grass type and purpose:
To read the answer in kilograms, convert the pounds:
The overseeding rate is half the new-lawn rate:
Worked examples
Example 1: seeding a new tall fescue lawn
A lawn is 50 ft long and 40 ft wide, and you are sowing tall fescue on bare soil (a new lawn, so the rate is 8 lb per 1,000 ft²):
So the lawn needs 16 lb of tall fescue seed — 7.257 kg. A 20 lb bag covers it with a little left over for patching.
Example 2: overseeding the same lawn
The same 2,000 ft² lawn is already established and just needs thickening. Overseeding tall fescue uses half the new-lawn rate, 4 lb per 1,000 ft²:
That is 8 lb of seed (3.629 kg) — half of what the same lawn would take from bare soil. Had the lawn been Kentucky bluegrass instead, the new-lawn rate of 2 lb per 1,000 ft² would have called for only 4 lb, and overseeding it just 2 lb.
Practical notes
- Measure the grass, not the plot. Subtract the house, the driveway, the patio, and the beds. Seeding a measured lawn area rather than the whole property is usually the difference between one bag and two. If you also need to work out turf, soil, or mulch for the same space, the sod calculator, the soil calculator, and the mulch calculator take the same length and width.
- Match the seed to the climate. Kentucky bluegrass, the fescues and perennial ryegrass are cool-season grasses, sown in late summer or early autumn in temperate regions. Bermuda is a warm-season grass, sown in late spring once the soil is properly warm.
- Resist over-seeding the rate. Doubling the seed does not double the lawn. Crowded seedlings compete for moisture and light, and a dense, spindly stand is more vulnerable to damping-off disease than a correctly sown one.
- Spread it in two passes. Set the spreader to half the rate and cover the lawn twice, walking the second pass at right angles to the first. It evens out the coverage far better than a single heavy pass and hides any spreader-lane stripes.
- Buy a little extra. Add 5–10% for the seed left in the hopper, the patch by the gate that always washes out, and the strip you will re-seed in spring.